At present, the methods for the determining thermal maturity of oil source rocks are mainly classified into two types: one type of method studies the parameters of the evolutionary characteristics of rock kerogens, such as vitrinite reflectance (Ro), rock pyrolysis, fluorescence alteration of multiple macerals (FAMM), the degree of spore-pollen carbonization, thermal alteration index (TAI), conodont alteration index (CAI), kerogen elemental composition and the like, which method, despite of relatively high accuracy, has drawbacks such as requirement of large sample amount, complex testing procedures and extended duration; the other studies the parameters of the evolution characteristics of soluble organic substances in oil source rocks, such as carbon preference index (CPI), odd-even predominance (OEP), biomarker compound parameters such as sterane and terpane isomerization ratio, which is mainly used for qualitative evaluation but hardly for quantitative analysis.
Vitrinite reflectance is observed by using the reflectance of organic particles and is closely related to the aromatization and polycondensation of the organic molecules; depending on the refractive index and absorption coefficient, it increases as the double bonds of carbon atoms increase, that is, it increases as the degree of aromatization of vitrinite components increases. The major limitation to vitrinite reflectance is that vitrinite components contribute poorly to the oil generation as compared to the liptinite components, while some source rocks having strong predisposition to oil generation are deficient in or contain very little vitrinite. Therefore, the application of vitrinite reflectance is limited.